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Smaller Solar System Bodies - Agreed Terms - March 2022

Rocks in the Solar System. Maybe we should call them sub-planetary objects? I do not want to get involved in geological differences between rocks and minerals. For those that do, I think the best distinction is similar to that between mixtures and compounds. Rocks (geologically) are mixtures of materials originating from the formation of the Solar System, sometimes directly, sometimes via subsequent aggregations, such as asteroids, having been involved in impacts. Minerals are the chemically defined ingredients.

That said, we are looking at names given to certain groups of these smaller Solar System bodies. Asteroids and Comets are treated separately.

To start with, we have a very general category – Trans Neptunian Objects or TNOs. Simple, in principle, anything beyond the orbit of Neptune at about 30 AU.

Trans-Neptunian object - Wikipedia › wiki › Trans-Neptunian_object

A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average ...
History · ‎Classification · ‎Physical characteristics · ‎Notable objects

Extreme trans-Neptunian object · ‎Resonant

Eight trans-Neptunian objects | The Planetary Society


There are many objects within this 30 AU demarcation, but, let’s these aside for the moment.

For the purpose of this survey, it includes anything in regular orbit smaller than a planet, excluding asteroids and planetary satellites. Comets are also excluded.

There are two main named regions beyond the orbit of Neptune – the Kuiper Belt and the Oort Cloud, and these give rise to heir named inhabitants – Kuiper Belt Objects or KBOs. Whilst the Kuiper Belt is generally stated as beginning at 30 AU (the orbit of Neptune), the outer limit has been put at a variety of distances. One NASA suggestion is about 55 AU.


The classical KBOs are on relatively low inclination, low eccentricity orbits between 40 and 47 AU [16]. Their nearly circular orbits, and especially those of a sub-group of dynamically "cold" classical KBOs (CCKBOs), indicate that they have experienced little perturbation (i.e., low eccentricity and inclination) since they formed in the protoplanetary nebula.

Imgur: The magic of the Internet

Kuiper Belt: In Depth | NASA Solar System Exploration

Classical Kuiper belt object

The Kuiper Belt: Objects at the Edge of the Solar System

Beyond Neptune, the Kuiper Belt is a vast region of icy objects called Kupier Belt Objects (KBOs), including dwarf planets Pluto, Eris, ...

The Oort Cloud has never been observed – not difficult to understand, as it is suggested as between at a variety of distances such as 3000 AU to 100,000 AU and as being distributed spherically around the Sun. At these distances, small (or even large) objects of low albedo (ability to reflect light) stand little chance of being seen from Earth. It has been suggested that there may be as many as 1012 icy objects out there, with the potential to become comets, if deflected to the inner Solar System.

100,000 AU translates to 1.58 light years (the conversion factor is to divide by 63241), which is almost half way to the nearest star, and it has even been suggested that, in regions of high stellar density, the Oort Cloud might form a continuous surrounding medium.

Oort Cloud – Wikipedia

Oort cloud - Wikipedia › wiki › Oort_cloud

The Oort cloud sometimes called the Öpik–Oort cloud, first described in 1950 by the Dutch astronomer Jan Oort, is a theoretical concept of a cloud of ...
Jan Oort · ‎Hills cloud · ‎Hill sphere · ‎Planetesimal

Then we have Scattered Disk Objects (SDOs). They are a subset of KBOs with large orbital eccentricities, typically with perihelia (closest Sun distances) between 30 and 48 AU, and aphelia (furthest distances from Sun) greater than 60 AU. They may have forced into such orbits by gravitational interactions with the giant planets early in the formation of the Solar System. They have orbital eccentricities as high as 0.8 and inclinations as high as 40o.


Plutinos are a group of TNOs that orbit in a 2:3 resonance with Neptune. This means that they make 2 orbits for every 3 made by Neptune. The dwarf planet Pluto is the largest member, other large members being Orcus and Ixion. Plutinos inhabit the inner Kuiper Belt and are about a quarter of known KBOs. They are also most of those objects with orbits in resonance with Neptune.

Ref: Plutino - Wikipedia

There are within this region, the dwarf planets, of which the best known is Pluto. The dwarf planet category was introduced by the IAU in 2006, largely because Pluto, having been defined a planet since 1930, was found to be one of potentially a large number of similar objects, starting with Eris, leading eventually from 9 ‘planets’ to thousands. Clearly a new definition was needed.

Dwarf planet - Wikipedia

Dwarf planet - Wikipedia › wiki › Dwarf_planet

A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, ...

List of possible dwarf planets

Smaller bodies inside the orbit of Neptune include the Centaurs, Trojans


Centaur (small Solar System body) - Wikipedia › wiki › Centaur_(small_Solar_...

In planetary astronomy, a centaur is a small Solar System body with either a perihelion or a semi-major axis between those of the outer planets.
Classification · ‎Discrepant criteria · ‎Ambiguous objects · ‎Orbits

Imgur: The magic of the Internet

In astronomy, a trojan is a small celestial body (mostly asteroids) that shares the orbit of a larger one, remaining in a stable orbit approximately 60° ahead of or behind the main body near one of its Lagrangian points L4 and L5. Trojans can share the orbits of planets or of large moons.

Trojan (celestial body) - Wikipedia
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I am retired early and interested in everything from the Big Bang (and before) from Cosmology to Astronomy and Planetary Sciences to Geology including the 'geology' of exoplanets and miscellaneous objects including asteroids. That covers quite a...
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